Generally, medical X-ray detectors employing a scintillating phosphor screen to absorb X-rays and produce light suffer the loss of spatial resolution due to lateral light diffusion in the phosphor screen. To reduce lateral light diffusion and maintain acceptable spatial resolution, the phosphor screens must be made sufficiently thin.
The spatial resolution and X-ray detection ability of an imaging apparatus are often characterized by the modulation transfer function (MTF) and X-ray absorption efficiency, respectively. Thin phosphor screens produce better MTF at the expense of reduced X-ray absorption. Usually, the coating density and the thickness of the phosphor screen are used in the design tradeoff between spatial resolution and X-ray absorption efficiency.
For example, the Lanex Fine and the Lanex Fast Back screens are two typical commercial screens, both manufactured by Eastman Kodak Co. Both are made of Gd2O2S(Tb) phosphor. The Lanex Fast Back screen is relatively thicker and absorbs X-rays more efficiently, but has lower resolution than the Lanex Fine screen. On the other hand, the Lanex Fine screen is thinner than the Lanex Fast Back screen, absorbs X-rays relatively less efficiently, but has higher resolution. The coating density of the Lanex Fine and the Lanex Fast Back screens are 34 mg/cm2 and 133 mg/cm2, respectively. The Lanex Fine and the Lanex Fast Back screens have X-ray absorption efficiencies of 24% and 63% (for 80 kVp, with tungsten target, 2.5-mm Al inherent filtration, and filtered by 0.5-mm Cu+1.0-mm Al) and MTF values of 0.26 and 0.04 at 5 c/mm, respectively.
Recently, digital flat panel X-ray imagers based upon active matrix thin film electronics have become a promising technology for applications such as diagnostic radiology and digital mammography. There are two types of X-ray energy conversion methods used in digital radiography (DR), namely, the direct and indirect method. In the direct method, the X-rays absorbed in a photoconductor are directly transduced into a charge signal, stored on the pixel electrodes on an active matrix array (AMA) and read out using thin film transistors (TFTs) to produce a digital image. Amorphous selenium (a-Se) is usually used as the photoconductor.
In the indirect method, a single phosphor screen is used to absorb X-rays and the resultant light photons are detected by an AMA with a single photodiode (PD) and a TFT switch at each pixel. The photodiode absorbs the light given off by the phosphor in proportion to the X-ray energy absorbed. The stored charge is then read out, like the direct method, using the TFT switch. Common phosphor materials include powder phosphors such as Gd2O2S(Tb) and structured phosphors such as CsI(Tl). Amorphous hydrogenated silicon (a-Si:H) is commonly used to form the photodiode and the TFT switch in the indirect method.
FIG. 1A shows a cross-section (not to scale) of a single imaging pixel 10 in a prior art a-Si-based flat panel imager used in the indirect method and FIG. 1B shows a schematic top-view of a plat panel imager 80 including an array of such pixels 10. Each imaging pixel 10 has a photodiode 70 and a TFT switch 71. A layer of X-ray converter (e.g., luminescent phosphor screen 12) is coupled to the photodiode-TFT array. Photodiode 70 comprises the following layers: a passivation layer 14, an indium tin oxide layer 16, a p-doped Si layer 18, an intrinsic a-Si:H layer 20, an n-doped Si layer 22, a metal layer 24, a dielectric layer 26, and a glass substrate 28. An X-ray photon path 30 and a visible light photon path 32 are also shown in FIG. 1A. As illustrated, when a single X-ray is absorbed by the phosphor, a large number of light photons are emitted isotropically. Only a fraction of the emitted light reaches the photodiode and gets detected.
As shown in FIG. 1B, the flat panel imager 80 consists of a sensor array 81 including a matrix of the a-Si n-i-p photodiodes 70 and TFTs 71. Gate driver chips 82 are connected to the blocks of gate lines 83 and readout chips are connected to blocks of data lines 84 and bias lines 85. Each of the data lines 84 has an associated charge amplifier 86. The amplifiers preferably include double correlated sampling circuits with programmable filtering (not shown), and are in communication with an analog multiplexer 87, which in turn communicates with an analog-to-digital converter (ADC) 88, to stream out the digital image data at desired rates.
The operation of the a-Si based indirect flat panel imager is known by those skilled in the art, and thus only a brief description is given here. Incident X-ray photons are converted to optical photons in the phosphor screen 12, and these optical photons are subsequently converted to electron-hole pairs within the a-Si:H n-i-p photodiodes 70. In general, a reverse bias voltage is applied to the bias lines 85 to create an electric field (and hence a depletion region) across the photodiodes and enhance charge collection efficiency. The pixel charge capacity of the photodiodes is determined by the product of the bias voltage and the photodiode capacitance. The image signal is integrated by the photodiodes while the associated TFTs 71 are held in a non-conducting (“off”) state. This is accomplished by maintaining the gate lines 83 at a negative voltage. The array is read out by sequentially switching rows of TFTs to a conducting state by means of TFT gate control circuitry. When a row of pixels is switched to a conducting (“on”) state by applying a positive voltage to the corresponding gate line 83, charge from those pixels is transferred along the data lines 84 and integrated by the external charge-sensitive amplifiers 86. The row is then switched back to a non-conducting state, and the process is repeated for each row until the entire array has been read out. The signal outputs from the external charge-sensitive amplifiers 86 are transferred to the analog-to-digital converter (ADC) 88 by the parallel-to-serial multiplexer 87, subsequently yielding a digital image. The flat panel imager is capable of both single-shot (radiographic) and continuous (fluoroscopic) image acquisition.
The conventional scintillating phosphor screen imaging panel has three basic components: a substrate of glass or other rigid, transparent material, a TFT layer formed on the substrate, and a phosphor layer containing the scintillator material. There would be advantages in simplifying the design of the imaging panel and reducing size, weight, and cost by eliminating components that are not directly involved in obtaining the image data.